Field of the Invention
This invention relates to electric heaters. More specifically, the invention relates to coiled heaters for nozzles on plastic injection molding equipment.
Plastic injection molding utilizes steel molds which are separable and have cavities into which molten plastic resin is injected under pressure. Often nozzles, which are utilized to inject the plastic resin into the mold cavities or in ducts leading to the cavities, must have supplemental heating to properly control the molten resin temperature as it is injected into the cavity. The supplemental heating may be provided by electric heaters placed on or at the nozzles. Such heaters will typically consist of a heater wire supported by insulative material contained within a length of metallic tubular sheath. The metallic tubular sheath may be wound into a coil which is secured to the nozzle. The coiled sheath containing the heater element wire is then contained within a housing or clamp and is secured to the nozzle.
For purposes of heat transfer from the heater to the nozzle, it is advantageous to provide radial compression between the heater and the nozzle. Prior art heaters have, for example, utilized outer tubes with an axial slit to secure the coiled tubular sheaths directly to the nozzle. Such clamps, as illustrated in U.S. Pat. No. 5,263,230 owned by the assignee of this application, utilize mechanisms to generate a tangential pulling force on the tube across the slit thereby reducing the diameter or the tubular shaped clamp and thereby radially compressing the coiled sheaths onto the nozzle. See also, U.S. Pat. No. 4,563,795 for a different tightening mechanism. Although these types of clamps work exceedingly well in many applications, it is at times desirable to have a heater with a minimal diametric cross-section. The mechanisms utilized to create the tangential pulling force extend radially outward from the heater clamp. In some applications it would be desirable to have the minimum possible cross sectional profile while still providing radial compression of coiled metallic sheath.
Other known clamps utilize an inner housing and outer housing with the coiled tubular sheaths intermediate said inner and outer housing. Such clamps have not heretofore provided effective radial compression of the coiled metallic sheath directly onto the nozzle or compression of the coiled metallic sheath on an inner by way of an actuatable clamping mechanism.